Reference: pmCreateGradientPixmap

pmCreateGradientPixmap(W, H, C1, C2, type) creates a W×H pixmap containing a linear gradient from the colour C1 to the colour C2. C1 and C2 are colour codes, and W, and H are positive integers. If type is "horizontal", a horizontal gradient is created. If type is "vertical", a vertical gradient is created.

Example: pmCreateGradientPixmap(100, 25, rgb(1, 0, 0), rgb(0, 0, 0), "horizontal")